Error upgrade the DigitalOcean One-Click Drupal Image of drupal8

I created a new Ubuntu Drupal 8.1.3 on 14.04,But I want to drupal upgrade 8.2.5.

drush up drupal

root@drupal-512mb-sfo2-01:/var/www/html/drupal/sites/default# drush up drupal
Command pm-update needs a higher bootstrap level to run - you will need to invoke drush from a more functional Drupal environmen
to run this command.
The drush command 'up drupal' could not be executed.
Drush was not able to start (bootstrap) the Drupal database.
Hint: This may occur when Drush is trying to:
* bootstrap a site that has not been installed or does not have a configured database. In this case you can select another site
with a working database setup by specifying the URI to use with the --uri parameter on the command line. See `drush topic
docs-aliases` for details.
* connect the database through a socket. The socket file may be wrong or the php-cli may have no access to it in a jailed shell
See http://drupal.org/node/1428638 for details.
Drush was attempting to connect to:
Drupal version : 8.1.3
Site URI : http://default
Database driver : mysql
Database hostname : 127.0.0.1
Database port :
Database username : drupal
Database name : drupal
PHP configuration : /etc/php5/cli/php.ini
PHP OS : Linux
Drush script : /usr/local/bin/drush
Drush version : 8.1.2
Drush temp directory : /tmp
Drush configuration :
Drush alias files :
Drupal root : /var/www/html/drupal
Drupal Settings File : sites/default/settings.php
Site path : sites/default
Sync config path : sites/default/files/config_zWDi3fOuGc5C_D6ebw8cglPgBBXQJRq5Aznm2bexOjgdUTFATTXRQY-BHxerNyJBiq3ucYBh4
Q/sync